In a case where heat is exchanged between a fluid and a dry air through a heat exchange surface, a dropwise condensation, a frost formation, or a freezing frequently occurs on a side of the heat exchange surface contacting air under the condition that a temperature of the heat exchange surface (referred to as a cooling surface hereinafter) is lower than that of air.
Here, conditions for occurrence of frost formation, or the condensation phenomenon are explained, with reference to FIG. 3. If a condition of water vapor in an atmosphere corresponds to a water-saturated atmosphere (including super-saturated state) under air temperature higher than 0° C., water droplets are generated by water vapor being condensed to condensation nuclei in an atmosphere, and then, falls and accumulates on the cooling surface, whereby water vapor is condensed to such accumulated water droplets by a repetition of the above growth and combining into one process to form into big droplets. When a gravity force exerting on such big droplets exceeds an adhesion force between the big droplets and the cooling surface, the big droplets flows (falls) down on the cooling surface.
If a condition of water vapor in an atmosphere corresponds to the water-saturated atmosphere (including super-saturated state) under air temperature between 0° C. and −40° C., super-cooled water droplets are generated by the water vapor being condensed to condensation nuclei in an atmosphere, and then, fall and accumulate on the cooling surface, whereby the super-cooled water droplets grow to be joined to each other, and then, become frozen, and as a result, the water vapor sublimates to the frozen ice particles to cause the formation of frost.
If a condition of water vapor in an atmosphere corresponds to an ice super-saturated atmosphere and does not correspond to the water-saturated atmosphere under air temperature between 0° C. and −40° C., ice crystals are generated by water vapor being sublimated to sublimation nuclei in the atmosphere, and then, fall and accumulate on the cooling surface, whereby water vapor are sublimated to such accumulated ice crystals to cause the formation of frost.
Now, the condensation or the sublimation phenomenon is explained in more detail. When moist air is cooled, water vapor in the atmosphere becomes super-saturated (referred to as a water super-saturated state) in which the water vapor cannot maintain its gas state any longer, so that the condensation phenomenon sets in. An air temperature at this state is referred to as the dew point. In addition, in a case where an ambient temperature is below 0° C., the water vapor can become either an ice super-saturated state or a water super-saturated state. This is because the amount of super-saturated water vapor under the ice state is smaller than that under the water state, the ice super-saturated phenomenon precedes over the water super-saturated phenomenon, so that the water vapor over the amount of the super-saturated water vapor emerges as ice crystals (referred to as ice crystal hereinafter) by sublimating to the ice crystal nuclei in the atmosphere. An air temperature at the stage is referred to as a freezing point. In this connection, if the water vapor is further cooled under a low temperature to become a water super-saturated condition where a condensation phenomenon sets in, like the case of the air temperature above 0° C., however, under the condition of the air temperature is below −40° C., the condensed droplets immediately become the super-cooled droplets without being frozen. An air temperature at the stage is also referred to as dew point, like a case of the air temperature above 0° C. The super-cooled droplets stochastically become frozen with time. Since the water vapor pressure of the ice is lower than that of the surroundings, water vapor positively sublimates to such an icy surface, whereby frost crystals P4 rapidly start to grow.
In addition, in a case where a condition of water vapor in the atmosphere corresponds to a water-saturated atmosphere (including super-saturated state) under the condition that the air temperature is below −40° C., the water vapor is caused to condense to the condensation nuclei in the atmosphere to immediately form into frozen particles, and then, frozen particles having fallen and accumulated on the cooling surface to form frost in a powder form. In this connection, if the temperature of the cooling surface is below −40° C., but the air temperature in the atmosphere is above −40° C. warmer than the cooling surface, the accumulated powder frost gets thick, and if the temperature of the surface of the frost layer becomes above −40° C. due to that it is exposed to the atmosphere, water vapor sublimates to the frost to cause the formation and the growth of the frost.
Further, in a case where a condition of water vapor in the atmosphere corresponds to an ice super-saturated atmosphere and does not correspond to the water-saturated atmosphere under the condition that the air temperature is below −40° C., the water vapor is caused to sublimate to sublimation nuclei in the atmosphere to immediately form into ice crystals, and then, the water vapor sublimates to ice crystals having fallen and accumulated on the cooling surface to form frost.
In this connection, the above explanation is based on the assumption that the condensation nuclei or the sublimation nuclei exist in the atmosphere within the temperature boundary layer near the cooling surface. However, since the condensation nuclei or the sublimation nuclei also exist on the cooling surface, the condensation or the sublimation phenomenon can directly occur on the cooling surface. This follows that, even if the super-saturated phenomenon does not occur in the air, the condensation or the sublimation phenomenon can occur on the cooling surface, only if the condition of the cooling surface corresponds to the surroundings.
The dew is a cause of a deterioration of a hygienic aspect such as generation of fungus, corrosion, electrical leak, or a smear of the heat exchange surface S, while the formation of the frost or the freezing is a cause for a decrease of the amount of heat exchange along with a thermal resistant layer caused by a liquid membrane on the heat exchange surface S upon the generation of dew, since a frost layer or an ice layer forms another thermal resistant layer upon the heat exchange and its physical thickness hinders an air-passage. Needless to say, if the frost or the ice melts, the problem is the same as in the case of the dropwise condensation generating dew occurs. Such being the case, conventionally, various kinds of technologies for defrosting or dehumidifying the heat exchange surface S has been adopted.
In this connection, a patent publication 1 discloses an agent for adjusting a humidity using multi-cellular material, or an agent for preventing dew.
More specifically, the agent for adjusting a humidity using multi-cellular material, or the agent for preventing dew, are constituted by agglomerating fine particles at a nano level without a gap between the particles being lost each of which particle does not include multi-cellular characteristics. In other words, multi-cellular material including an empty hole at a nano level between fine particles is adopted, so that a multi-cellular structure including a distribution of fine holes in which a diameter of fine hole ranges between 1 nm and 10 nm. Based on a capillary condensation theory by Kevin, the amount of adsorbing water vapor increases at the range of relative humidity of between 75% and 93%. More concretely, an isothermic adsorbing curve rises near about 80%, and the amount of adsorbing water vapor between relative humidity of 75% and 93% is about 12 mass %, so that water vapor adsorbed between the relative humidity of 75% and 93% is emitted at the relative humidity 70%, whereby an ability for preventing dew is recovered, under the isothermic adsorbing curve.
By such an agent for adjusting a humidity using multi-cellular material, or an agent for preventing dew, water vapor in moist air which causes dew is adsorbed, while at the same time, the ability for preventing dew can be recovered by adsorbed water vapor being emitted, so that the agent can be repeatedly used. In addition, water vapor in the moist air can be caught due to the diameter of the fine hole being between 1 nm and 10 nm. However, in a case where super-cooled condensed droplets are generated in the moist air under the condition that the temperature of moist air is below 0° C., humidity cannot be adjusted, or the generation of dew cannot be prevented by catching super-cooled condensed droplets, since the diameter of super-cooled condensed droplets is at least 1 μm.
In this respect, it has been desired to realize a method of maintaining the maintenance-free heat exchange surface by preventing mass transfer on the heat exchange surface whose temperature largely differs from the surroundings, in case of a device for cooling moist air for a refrigerator processing moist air with temperature of below 0° C.
On the other hand, in a case where moist air is cooled to below 0° C. by a device for cooling moist air, in particular, or in a case where heat is adsorbed from the moist air by a LNG vaporizer, not dew, but frost formation or freezing can occur on the cooling surface which constitutes the heat exchange surface.
In such a case, a frost layer becomes a thermal resistant layer, because of its low thermal conductivity, or grown frost can block a passage of the moist air which is a target to be cooled, so that an efficiency of exchanging heat can decrease, on the whole.
In this respect, a patent publication 2 discloses a heat exchanger which can utilize a solidification heat, while at the same time, can continuously operate for a long time by making it easy to mechanically remove frost.
More specifically, this heat exchanger is the one which can adsorb heat from the moist air and includes fine concave and convex portions on its surface. On an upper surface of the convex portion, a flat portion with a minimum width being between 100 μm and 500 μm is formed, and a minimum width of the concave portion is between 100 μm and 1000 μm. Frost crystals P4 can vertically grow on the flat portion of the upper surface of the convex portion, by providing the convex and concave portions on the surface of the heat exchanger. Since the frost crystals P4 grow on the convex portions, while a gap is formed around the concave portions, the frost crystals P4 in a comb-teeth form are formed. Such a comb-teeth form is structurally weak, the frost crystals P4 can be readily removed by a mechanical device such as a brush, a scraper, etc. This allow for the heat exchanger to be continuously operated for a long time, while at the same time to utilize the solidification heat.
Further, a patent publication 3 discloses a member for preventing a frost formation. More specifically, in this member, a water repellant portion and a hydrophilic portion whose hydrophilic property is higher than the water repellant portion are formed in a predetermined pattern.
A frost is difficult to form on the water repellant portion due to its high water repellant property, while a frost is easy to form on the hydrophilic portion. Accordingly, a frost on the hydrophilic portion grows until its size becomes the one which cannot resist on an air flow, and then, it collapses, since a frost cannot grow on the water repellant portion, while a frost can largely grow on the hydrophilic portion. Such a growth and a collapse of frost is repeated.
As described above, a frost formation can be suppressed by promoting a repetition of the growth and the collapse of frost by means of the formation of the water repellant portion and the hydrophilic portion in a predetermined pattern.
However, in a case where a frost formation is prevented by the process or the treatment of the heat exchange surface, as disclosed by the patent publications 2 and 3, a frost formation can inevitably occur with time, so that a state in which a frost is not formed cannot be maintained for a long time.
On the other hand, since the situation in which frost is formed can vary, in accordance with the conditions on the temperature and the humidity of the coolant or the moist air, or the variation of the state in which the moist air flows, it is difficult to meet the variation of such conditions.
Further, although it is possible to accelerate a sensible heat exchange, since frost formation of the moist air on the cooling surface can be prevented, a latent heat exchange (solidification heat) involved by change of phase of water vapor is excluded, so that the method of exchanging heat in total is not necessarily improved.
In this connection, a patent publication 4 discloses a device for reducing frost formation on a cooler. More specifically, this device is disposed near the heat exchanger for cooling including a heat transfer tube and a plurality of fins each of which is attached on the heat transfer tube, and includes a jetting means including a plurality of nozzles disposed perpendicular, or parallel to the direction in which planes of the fins extend, and a driving means for driving the jetting means in a reciprocal manner. The jetting means moves parallel or perpendicular to the direction in which planes of the fins extend to jet the moist air. The plurality of nozzles arranged in one row and move parallel or perpendicular to the direction in which planes of the fins extend to jet the moist air. The moist air is jetted to the entire area of the fins of the heat exchanger for cooling by discharging the moist air along the surface of the fins of the cooler, so that water droplets in a super-cooled state before they are formed into frosts and the frozen frost can be removed by exerting a fluid pressure on the frost formed on the surface of the fin, since frost formation can be reduced by a small amount of the moist air without halting the operation of the cooling device, the efficiency of the cooling operation can be maintained at a high level, and the cost for preventing frost formation and removing the frost can be reduced.
However, the device for reducing frost formation for the cooler forcibly removes the frost by jetting moist air to the frost formed on the surface of the fin, so that it neither prevents frost formation, nor utilizes the frost formed on the surface of the fin. In addition, maintenance has to be carried out in such a way that the formed frost does not block an opening of the nozzle, since the device for reducing frost formation is disposed near the heat exchanger for cooling.
In this respect, patent publications 5 and 6 disclose a net for removing iced frost or iced snow which removes snow from a wind-shield of an automobile, in a case where ice or frost is adhered to the wind-shield of the automobile, or in a case where snow is accumulated thereon.
More specifically, this net for removing iced frost or iced snow is constituted by wires with a predetermined width arranged in a planar mesh with a predetermined width and is directly laid on the wind-shield of the automobile.
By such a net for removing iced frost or iced snow, ice, frost, or snow accumulated on the wind-shield through opening portion of the mesh can be removed by pulling or removing the net which has become in one piece with the ice, or the frost formed in the opening portions of the mesh, or the snow accumulated in the opening portions of the mesh.
Such being the case, since the ice, frost, or snow to be removed and the net become in one piece, the width of the wire is determined in accordance with the thickness of the formed ice, frost, or snow, while the width of the mesh is determined in accordance with the adhesion force of the wires to the formed ice, frost, or snow.
In more detail, if the thickness of the ice, frost, or the snow is about 3 millimeter, the width of the wire is set to be between 2 millimeter and 6 millimeter, while the width of the mesh is set to be between 10 millimeter and 50 millimeter (patent publication 5). If the thickness of the ice, frost, or the snow is below 2 millimeter, the width of the wire is set to be between 0.5 millimeter and 2 millimeter, while the width of the mesh is set to be between 1 millimeter and 10 millimeter (patent publication 6).
In either of the above cases, the net for removing iced frost or iced snow merely removes iced frost or iced snow by pulling or removing the net which has been simply formed and has become in one piece with the iced frost or iced snow formed on the wind-shield of the automobile, like a case where the frost is not formed on the wind-shield of the automobile which is under a roof of a parking facility.
As described above, in the conventional heat exchange surface, the heat exchange surface cannot be maintained for a long time, and maintenance of the heat exchange operation on the cooling surface becomes difficult with time.
In short, a technical idea in which the condensation or frost formation phenomenon is caused separately from the cooling surface is neither suggested nor disclosed in the conventional heat exchange surface.    Patent Publication 1: Japanese Patent Publication No. 4599592    Patent Publication 2: Japanese Patent Laid-open Publication 2012-82989    Patent Publication 3: Japanese Patent Laid-open Publication 2003-240487    Patent Publication 4: Japanese Patent Laid-open Publication 2008-64326    Patent Publication 5: Japanese Utility Model Publication No. 3160488    Patent Publication 6: Japanese Patent Publication No. 4224121